In the automotive industry, including trucks, bushings are used in the suspension systems. More recently bushings are also being used to anchor stabilizer arms. Initially the bushings consisted of an annular metal-lined opening in which a pin rotated through an angle up to about 90.degree.. The metal-to-metal contact permitted corrosion of the bushing and also wear due to metal-to-metal movement. The bushing also required lubrication. An additional disadvantage of the all-metal bushing was that vibrations were transmitted through the bushings. In order to reduce the vibration transmitted through the metal bushing, it was replaced by a three-piece bushing comprising a pair of spaced concentric metal members or sleeves separated by an annular sleeve or member of rubber. The stabilizer arm shaft or pin mounted in the suspension system rotated in the inner concentric metal member, and in many instances rotation in the inner concentric metal member (inner) perpetuated many of the problems inherent in bushings having relative movement between two metal members.
To eliminate the problem of relative movement between two metal members, a shaft or pin was fixed to the inner concentric metal member of the bushing (inner) resulting in relative movement between the inner and the outer spaced concentric member (outer). This relative movement generates a stress in the rubber annular member producing a strain in the rubber member. The constant of proportionality which relates stress and strain of a bushing is defined as the dynamic spring rate. From a load-deformation standpoint, a softer bushing (one with a low spring rate) is expected to fail faster on fatigue than a harder bushing of a similar composition, when tested at equal load condition.
In addition to meeting a specified spring rate, it is also desirable that the bushing have good vibration damping properties and a long fatigue life. Fatigue life refers to the ability to withstand a large number of rotary oscillations.